1. Field of the Invention
The present invention relates to reduction of graininess in a case where color printing is performed by using plural kinds of coloring agents of different colors.
It should be noted that the present invention is applicable to all machines which perform printing by using a printing media such as a paper sheet, a cloth, a leather, a nonwoven fabric, an OHP sheet, a metal and the like. As the concrete machines to which the present invention is applicable, for example, business machines such as a printer, a copying machine, a facsimile machine and the like, and industrial-production machines such as a printing machine, a printing press and the like can be cited.
2. Related Background Art
As information processing machines such as a copying machine, a word processor, a computer and the like, and communication machines have been widespread, for example, an ink-jet printing apparatus has been also widespread as one of apparatuses for printing and outputting information to be processed by these machines. In the printing apparatus of such a type, a printhead on which plural ink discharge nozzles are integrally arranged is generally used to increase printing speed. Further, recently, a printing apparatus which uses printheads respectively for different colors has been widespread to cope with color printing.
In such an ink-jet printing method, ink being a recording liquid is discharged from an ink discharge outlet of the printhead, the discharged ink is hit against the printing medium such as a sheet or the like, a dot is formed by the hit ink, and the printing is thus performed. This method has an advantage of low noise because such a noncontact printing system as above is adopted. Further, a nozzle arrangement on the printhead can be easily made dense, whereby it is possible to achieve forming of a higher-resolution printed image and higher-speed printing. Also, this method has an advantage that colorization can be easily achieved by relatively simple head structure, whereby the apparatus itself can be minimized and simplified. In addition, any special process such as development, fixation or the like need not be performed to the printing medium such as a common sheet or the like, and a high-quality image can be thus obtained cheaply. Since there are such a lot of advantages, the ink-jet printing method is the printing method widely widespread in recent years. Besides, also making of high resolution and speed-up have been required more and more along with such the colorization.
Incidentally, particularly in the ink-jet printing method, when a color image is printed by using inks of three colors (i.e., C (cyan), M (magenta) and Y (yellow) inks) or inks of four colors (i.e., C, M, Y and K (black) inks), several methods have been known so as to try to form a multigradation image.
One of these methods has been adopted in a lot of conventional printers. In this method, the size of dots to be formed on a sheet by discharged ink is constant, and the gradation of the image to be printed is represented by changing dot density (a dot appearance frequency for each unit area). In another method, dot density for each unit area is made variable by adjusting the diameter of dots to be formed on a sheet.
Recently, since micromanipulation technology of the head forming ink droplets has advanced, the number of dots capable of being formed for a predetermined length (i.e., dot density (dpi: dots per inch)), a variable range of the dot diameter and the like have improved every year. However, in the ink-jet printer, printing density (i.e., resolution) has stagnated to about 300 dpi to 1200 dpi and an ink droplet diameter has stagnated to about several tens of microns. That is, the expressiveness of the ink-jet printer is still lagged behind as compared with a silver bromide camera (it is said that resolution of several thousand dots per inch can be obtained on a film).
In case of the ink-jet printer, the dots are sparsely formed particularly in the area where the density is low in the image to be printed, i.e., the area where the density of the dots to be formed is low, whereby the image that so-called graininess stands out might be formed. Even if the density in the printing is constant, the positions where the dots are formed are generally different from others according to a binarization method such as a dither method or the like. Besides, the states that the dots are scattered are also different from others. For this reason, even if the dots are sparsely formed in the area where the dot density is low, there is a case where the dots are formed comparatively biasedly and thus the dots might not be uniformly distributed. In this case, the above graininess becomes especially remarkable.
Of course, irrespective of such a dot distribution, the graininess becomes remarkable as optical reflection density of the formed dots becomes higher or lightness thereof becomes lower.
With respect to such problems of the graininess as above, conventionally, a binarization process, e.g., a halftone process for the printer, has been devised for the problem of the bias of dot distribution. Concretely, the graininess has been reduced by not biasing the dot distribution for each color as much as possible in the area where the printing density is low.
Further, it has taken countermeasures, such as simple use of a low-density ink (e.g., dyes), against the basic problem of the graininess due to the high optical reflection density of the dots.
However, there is a case where the graininess of the dots still stands out even if such the conventional countermeasures as above are adopted. That is, with respect to the former problem, even if the binarization process is devised not to bias the dot distribution when the color printing is performed, such the binarization process itself is performed independently for each color. Thus, there is a case where a viewer feels the graininess because, e.g., the respective dot distributions of cyan and magenta mutually appear biasedly. For such a problem, a trial not to mutually bias the dot distributions of cyan and magenta has been performed, but the reduction of the graininess does not suffice when the dot diameter is relatively large.
Further, with respect to the latter problem, the method of reducing the graininess by using the low-density ink such as the dyes or the like naturally means that a low-density ink must be prepared besides each color ink, whereby there is some fear that costs of the printing apparatus resultingly increase.
The present invention has been made to solve the above conventional problems, and an object thereof is to provide an image processing method which can reduce graininess on a printed image by using a secondary color with relatively simple structure, a printing apparatus which uses this method, and a storage medium which stores a program to execute this method.
In order to achieve the above object, the present invention provides an image processing method which generates, based on data to be printed, printing data of plural different coloring agents used in a printing apparatus, the method comprising:
a step of generating first data of a secondary color for predetermined two of the plural different coloring agents based on the data to be printed, generating second data of the secondary color based on printing data of light black obtained for a black coloring agent being one of the plural different coloring agents, and thus generating printing data of the secondary color by synthesizing the first data and the second data; and
a step of generating printing data in which at least a part of the printing data of the predetermined two coloring agents has been replaced by the generated printing data of the secondary color.
Further, another aspect of the present invention provides a printing apparatus which performs printing on the basis of printing data of plural different coloring agents generated based on data to be printed, the apparatus comprising:
printing means for performing the printing of respective colors by using the plural different coloring agents, on the basis of the printing data; and
supply means for supplying, to the printing means, the printing data which is generated by
a secondary color generation process of generating first data of a secondary color for predetermined two of the plural different coloring agents based on the data to be printed, generating second data of the secondary color based on printing data of light black obtained for a black coloring agent being one of the plural different coloring agents, and thus generating printing data of the secondary color by synthesizing the first data and the second data, and
a data generation process of generating printing data in which at least a part of the printing data of the predetermined two coloring agents has been replaced by the generated printing data of the secondary color.
As described above, the printing data of the secondary color is generated for the predetermined two coloring agents among the plural different kinds of coloring agents used in the printing apparatus, the printing data in which the part of the printing data of the predetermined two coloring agents has been replaced by the generated printing data of the secondary color is generated, the printing data is generated at least to use the above secondary color instead of the light black coloring agent (i.e., the black coloring agent of which black density is low), and the printing is performed on the basis of these generated printing data. Therefore, even if the data to be printed should be essentially printed by using the dots formed by the predetermined two coloring agents, at least the part of such the printing can be performed by using the secondary color. Further, the area which should be essentially printed by the light black coloring agent can be printed by the above secondary color. Thus, particularly in the low-density part or the high-lightness part of the image to be printed, it is possible to reduce the area where the dots of the predetermined coloring agents are formed mutually biasedly. Also, the secondary color can be substituted in the printing of the light black part of the image.